[PATCH v2 2/2] selinux: add basic filtering for audit trace events

[Thiébaud Weksteen]

From: Peter Enderborg <peter.enderborg@xxxxxxxx>

This patch adds further attributes to the event. These attributes are
helpful to understand the context of the message and can be used
to filter the events.

There are three common items. Source context, target context and tclass.
There are also items from the outcome of operation performed.

An event is similar to:
           <...>-1309  [002] ....  6346.691689: selinux_audited:
       requested=0x4000000 denied=0x4000000 audited=0x4000000
       result=-13 ssid=315 tsid=61
       scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023
       tcontext=system_u:object_r:bin_t:s0 tclass=file

With systems where many denials are occurring, it is useful to apply a
filter. The filtering is a set of logic that is inserted with
the filter file. Example:
 echo "tclass==\"file\" && ssid!=42" > events/avc/selinux_audited/filter

This adds that we only get tclass=file but not for ssid 42.

The trace can also have extra properties. Adding the user stack
can be done with
   echo 1 > options/userstacktrace

Now the output will be
         runcon-1365  [003] ....  6960.955530: selinux_audited:
     requested=0x4000000 denied=0x4000000 audited=0x4000000
     result=-13 ssid=315 tsid=61
     scontext=system_u:system_r:cupsd_t:s0-s0:c0.c1023
     tcontext=system_u:object_r:bin_t:s0 tclass=file
          runcon-1365  [003] ....  6960.955560: <user stack trace>
 =>  <00007f325b4ce45b>
 =>  <00005607093efa57>

Note that the ssid is the internal numeric representation of scontext
and tsid is numeric for tcontext. They are useful for filtering.

Signed-off-by: Peter Enderborg <peter.enderborg@xxxxxxxx>
Reviewed-by: Thiébaud Weksteen <tweek@xxxxxxxxxx>
---
v2 changes:
- update changelog to include usage examples

 include/trace/events/avc.h | 41 ++++++++++++++++++++++++++++----------
 security/selinux/avc.c     | 22 +++++++++++---------
 2 files changed, 44 insertions(+), 19 deletions(-)

diff --git a/include/trace/events/avc.h b/include/trace/events/avc.h
index 07c058a9bbcd..ac5ef2e1c2c5 100644
--- a/include/trace/events/avc.h
+++ b/include/trace/events/avc.h
@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
- * Author: Thiébaud Weksteen <tweek@xxxxxxxxxx>
+ * Authors:	Thiébaud Weksteen <tweek@xxxxxxxxxx>
+ *		Peter Enderborg <Peter.Enderborg@xxxxxxxx>
  */
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM avc
@@ -12,23 +13,43 @@
 
 TRACE_EVENT(selinux_audited,
 
-	TP_PROTO(struct selinux_audit_data *sad),
+	TP_PROTO(struct selinux_audit_data *sad,
+		char *scontext,
+		char *tcontext,
+		const char *tclass
+	),
 
-	TP_ARGS(sad),
+	TP_ARGS(sad, scontext, tcontext, tclass),
 
 	TP_STRUCT__entry(
-		__field(unsigned int, tclass)
-		__field(unsigned int, audited)
+		__field(u32, requested)
+		__field(u32, denied)
+		__field(u32, audited)
+		__field(int, result)
+		__string(scontext, scontext)
+		__string(tcontext, tcontext)
+		__string(tclass, tclass)
+		__field(u32, ssid)
+		__field(u32, tsid)
 	),
 
 	TP_fast_assign(
-		__entry->tclass = sad->tclass;
-		__entry->audited = sad->audited;
+		__entry->requested	= sad->requested;
+		__entry->denied		= sad->denied;
+		__entry->audited	= sad->audited;
+		__entry->result		= sad->result;
+		__entry->ssid		= sad->ssid;
+		__entry->tsid		= sad->tsid;
+		__assign_str(tcontext, tcontext);
+		__assign_str(scontext, scontext);
+		__assign_str(tclass, tclass);
 	),
 
-	TP_printk("tclass=%u audited=%x",
-		__entry->tclass,
-		__entry->audited)
+	TP_printk("requested=0x%x denied=0x%x audited=0x%x result=%d ssid=%u tsid=%u scontext=%s tcontext=%s tclass=%s",
+		__entry->requested, __entry->denied, __entry->audited, __entry->result,
+		__entry->ssid, __entry->tsid, __get_str(scontext), __get_str(tcontext),
+		__get_str(tclass)
+	)
 );
 
 #endif
diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index b0a0af778b70..7de5cc5169af 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -705,35 +705,39 @@ static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
 {
 	struct common_audit_data *ad = a;
 	struct selinux_audit_data *sad = ad->selinux_audit_data;
-	char *scontext;
+	char *scontext = NULL;
+	char *tcontext = NULL;
+	const char *tclass = NULL;
 	u32 scontext_len;
+	u32 tcontext_len;
 	int rc;
 
-	trace_selinux_audited(sad);
-
 	rc = security_sid_to_context(sad->state, sad->ssid, &scontext,
 				     &scontext_len);
 	if (rc)
 		audit_log_format(ab, " ssid=%d", sad->ssid);
 	else {
 		audit_log_format(ab, " scontext=%s", scontext);
-		kfree(scontext);
 	}
 
-	rc = security_sid_to_context(sad->state, sad->tsid, &scontext,
-				     &scontext_len);
+	rc = security_sid_to_context(sad->state, sad->tsid, &tcontext,
+				     &tcontext_len);
 	if (rc)
 		audit_log_format(ab, " tsid=%d", sad->tsid);
 	else {
-		audit_log_format(ab, " tcontext=%s", scontext);
-		kfree(scontext);
+		audit_log_format(ab, " tcontext=%s", tcontext);
 	}
 
-	audit_log_format(ab, " tclass=%s", secclass_map[sad->tclass-1].name);
+	tclass = secclass_map[sad->tclass-1].name;
+	audit_log_format(ab, " tclass=%s", tclass);
 
 	if (sad->denied)
 		audit_log_format(ab, " permissive=%u", sad->result ? 0 : 1);
 
+	trace_selinux_audited(sad, scontext, tcontext, tclass);
+	kfree(tcontext);
+	kfree(scontext);
+
 	/* in case of invalid context report also the actual context string */
 	rc = security_sid_to_context_inval(sad->state, sad->ssid, &scontext,
 					   &scontext_len);
-- 
2.28.0.236.gb10cc79966-goog